Commercial automatic transmissions generally include a hydraulic retarder or hydrodynamic retarder that is used during vehicle braking. These devices absorb energy during vehicle braking and thereby conserve the service brake of the vehicle particularly in downhill operations. The hydrodynamic retarder converts the energy of the driveline into heat and dissipates it using the transmission oil cooling system.
A typical retarder control system consists of a flow valve to direct oil to and from the retarder and a regulator valve to control the pressure in the retarder cavity. The regulator valve generally controls the discharge oil from the retarder. By controlling the retarder pressure, the energy dissipation is also controlled.
These systems work very well under normal operating conditions; however, it is possible for the regulator valve to remain in the open or regulating position due to contamination or other elements that might be found in the fluid. With this occurring, the retarder cavity can be subjected to higher than needed pressures, which results in higher than normal absorption of energy and higher oil temperatures.
In many of the current retarder controls, it is common to use a relay valve to conduct fluid from the regulator valve to the retarder flow valve. The relay valve is a conventional off-on valve that does not affect the pressure within the system.